In the usual tube furnace for the thermal cracking of organic compounds, a duct system consisting of a single tube in the form of a meander, coil or other undulating arrangement of tube passes traverses a furnace chamber in the walls of which burners are mounted, the tube being of constant cross section from its inlet end to its outlet end.
Such tube furnaces have been provided heretofore for the thermal cracking of hydrocarbons such as naphtha and use burners arrayed on the walls of the combustion chamber in which the tube is arranged. The tube generally is looped to provide the numerous passes which are desirable in a small volume.
Above the combustion chamber a convection zone is commonly provided in which the hydrocarbon is, before entering the tube coil of the combustion chamber, preheated by the combustion gases to a temperature below that at which significant cracking occurs.
The cracking process is dependent upon the temperature to which the hydrocarbon is heated and the residence time of the hydrocarbon is the combustion chamber and hence the yield of cracking products, such as ethylene, is found to depend upon the increased temperature and a short residence time.
The short residence time is desirable to prevent secondary reactions of the primary desired product, the secondary reactions resulting in the formation of high-molecular-weight products. While the simple solution to the problem of increasing the yield of the desired product might appear to be the shortening of the residence time and the increasing of the temperature, it is found that the temperature has a maximum above which further increase in temperature will not increase the yield of the desired product. Concommittently the residence time has a lower limit below which cracking diminishes or does not appreciably occur. Hence a minimum residence time and a maximum cracking temperature must be observed.
With conventional systems of the character described, however, the throughput is limited and difficulties in controlling the system are encountered because of the criticality of the parameters at which the system must operate.